In motor vehicle power steering systems such as described in U.S. Pat. Nos. 3,022,772 and 4,454,801, issued Feb. 27, 1962 and Jun. 19, 1984, respectively, and assigned to the assignee of this invention, the output of an engine-driven hydraulic pump is throttled at orifices of a proportional control valve to create a steering assist boost pressure in a high pressure fluid circuit upstream of the orifices. The boost pressure reacts against a piston of a fluid motor which, in turn, applies a force supplementing manual steering effort. In the proportional control valve, manual effort applied at a steering wheel rotates a valve spool relative to a valve body against an elastic restoring force of a torsion bar between the two to establish the flow area of the throttling orifices. In the absence of manual effort, the torsion bar locates the valve spool in an open-center position in which the high pressure circuit communicates with a reservoir and fluid circulates freely from the pump to the reservoir at relatively low pressure. To achieve a scheduled relationship between boost pressure and manual effort regardless of pump speed, the engine-driven hydraulic pump has a constant flow valve which depends for its operation on a flow restriction in the high pressure circuit. In the open-center position of the valve spool in which fluid circulates freely in the high pressure circuit, the flow restriction of the constant flow valve needlessly consumes pump energy and, therefore, reduces engine fuel economy. A motor vehicle power steering system according to this invention is an improvement relative to the system described in the aforesaid U.S. Pat. Nos. 3,022,772 and 4,454,801.